Intaglio is a secular printing technique that goes back to the 15th Century which consists in printing patterns by means of an engraved printing plate (or intaglio printing plate) which is inked on its surface, then wiped before printing in order to contain the applied ink or inks within the engravings of the plate. The duly inked printing plate is applied against the paper to be printed in a press exerting sufficient pressure on the paper against the printing plate for the ink of the engravings to be transferred to the paper. The result is a document comprising a printed pattern reflecting the engraved patterns with an additional relief or embossing, matching the printed pattern, which reflects the depth of the engravings and which can be recognized by touch.
For a long time, the intaglio printing plates were commonly engraved by hand in a plate made of soft metal, for example copper, brass or any other metal or appropriate metal alloy. The tools used by the engraver typically comprise burins or punches, the ends of which are sharpened and adapted to the desired dimensions of the patterns to be engraved, said patterns mainly consisting of lines and curves dimensionally modulated according to the action applied by the engraver to produce variations in tones reproducing the halftones of the image to be engraved (for example a portrait).
More recently, photolithographic techniques have been proposed to facilitate the transfer of images to the printing plates, as well as computer-assisted engraving methods.
The intaglio printing technique is notably used in the field of security printing, in particular for the printing of banknotes, intaglio printing remaining one of the techniques that is most difficult to counterfeit without appropriate equipment.
In the context of security printing, in particular the printing of banknotes, techniques have been developed to assist the engraver in his work, notably to reduce the engraving time required and the time to produce printing plates. The approach adopted until very recently involved producing a single engraved original (engraved by hand or assisted by technical engraving means) representing a single document to be printed, and in replicating this original as many times as necessary to produce a printing plate comprising several identical replicas of said original. According to this approach, the engraving technique employed follows that of an engraver, that is, each pattern is engraved according to the plot of the line or curve to be made, that is vectorially (see, for example, International application WO 97/48555).
The present applicant has proposed a new approach for the production of intaglio printing plates, an approach that is described in International application WO 03/103962 (incorporated herein by reference in its entirety). This method consists in generating a set of three-dimensional digital data consisting of pixels each representative of an individual point to be engraved in the surface of the plate to be engraved, the engraving being done pixel by pixel on the basis of said three-dimensional digital data. According to this technique, a printing plate can be directly engraved. Alternatively, a printing plate precursor (advantageously a metal plate supporting a layer of polymer) can be engraved. In the latter case, it is the polymer layer that is engraved and the duly formed precursor is then used to make printing plates by electroplating. The engraving is, moreover, advantageously done by laser.
Unlike the preceding approach, a whole plate can be engraved in a single phase, without the tedious process of replicating an original. According to the technique described in the International patent application WO 03/103962, the replication of the original on the plate is done digitally, this making it possible in particular to compensate the distortions of the paper during the intaglio printing process, a compensation that was quite simply impossible by using the techniques employed previously.
A considerable advantage of the abovementioned technique lies in the fact that it is essentially independent of the complexity of the patterns to be engraved, whereas the prior techniques are dependent on the level of complexity of the patterns to be engraved.
FIG. 1 shows a halftone image representative of the state of the art which illustrates a portrait of Jules Verne and which is formed by a set of patterns consisting of lines and curves dimensionally modulated to produce the variations in tones reproducing the halftones of the image. The illustrated patterns are produced according to a conventional intaglio rendering technique. As can be see in FIG. 1, different parts of the portrait are reproduced by means of various combinations of lines and curves. Thus, the parts A of the portrait representing the skin on the forehead, the cheeks, the nose, are typically represented by means of a set of crossed lines and curves, with the possible addition of a dot or a line segment (commonly called “interlines” or “inner dots”) inside the lozenge-shaped spaces left between the crossed lines and curves. However, the hair and beard B are represented by a set of essentially parallel lines and curves. Finally, the eyes C are represented by a set of segments of concentric circles. All these representations, and others, are thus combined to reproduce various characteristic parts of the halftone image to be represented.
With regard to the conventional intaglio rendering technique illustrated in FIG. 1, a distinction should be drawn between this rendering technique and the photogravure (or rotogravure) rendering techniques which use regular screens of which the dot is dimensionally modulated to reproduce the halftones of the image. Although these techniques make it possible to reproduce halftone images, they have the disadvantage of eliminating any expressive force in the way with which the image is reproduced, rendering the image synthetic and lifeless, and equally degrading the level of security of such representations that can relatively easily be reproduced. Furthermore, photogravure employs inks that are comparatively more fluid than the intaglio inks and the printing method itself generates no relief on the printed document. Pattern generation methods for photogravure are, for example, described in document WO 83/00570. A device for the transposition by mechanical engraving of such patterns is for example described in publication U.S. Pat. No. 5,675,420.
From the security point of view, intaglio printing already in itself presents a high resistance to counterfeiting because of its specific features (relief, tactility, thickness of printed ink, etc.) which are difficult to reproduce with conventional means.
One way of increasing the level of security of intaglio prints for example involves incorporating microstructures that cannot be recognized by the naked eye in the printing itself. One solution notably involves structuring one of the lines or curves in the image in the form of a positive or negative microtext. The incorporation of such a microtext in a portrait is for example used in the American 100-Dollar banknote where the neckline of the portrait of the president Benjamin Franklin is structured to transcribe the expression “UNITED STATES OF AMERICA”. A proposition along these lines also features in International application WO 02/20268.
However, in this International application WO 02/20268, it is simply a question of superimposing, in positive or in negative, fine structures (for example microletters or geometrical symbols) on one or more lines or curves of a halftone image which is reproduced according to a conventional intaglio rendering technique. This results in positively or negatively structured lines, mirroring the practice applied to the abovementioned American 100-Dollar bill. This solution does not globally modify the rendering of the halftone image, which rendering is provided solely by a set of patterns consisting of dimensionally modulated lines and curves, where appropriate structured as indicated hereinabove.
Other solutions for increasing the security of the intaglio prints have been proposed. U.S. Pat. No. 5,018,767 for example proposes superimposing on the image a grid made up of parallel lines, the spacing of which is chosen to interfere with the operation of scanners and photocopiers.
Other solutions involve incorporating latent images in the intaglio print, that is, images that are revealed only under certain viewing conditions. Such solutions are notably proposed in publications U.S. Pat. No. 4,033,059 and U.S. Pat. No. 4,588,212. These solutions are, however, difficult to incorporate as such in halftone images without affecting the visual appearance of the latter.
There is therefore still a need to increase the level of security of the intaglio prints that represent halftone images.